Inspired by the channel-to-channel mapping, the authors of [1]} verified that the channel extrapolation among different terminals at any position is possible, and then applied this mapping concept to the distributed (cell-free) massive MIMO. With the key idea of [1]}, one can employ a subset of terminals to infer the information for the other terminals, which is named \(\mathrm {\it terminal~extrapolation}\) ; this is illustrated in Fig. REF .
If the terminals are close or are in a similar environment, then the terminal extrapolation is feasible and provides enough accuracy. However, with the increase of the distance between terminals, the number of common scattering objects becomes smaller and the latent relation becomes weaker, which deteriorates the performance of the terminal extrapolation.
In addition, this mapping would be closely related with the link conditions, such as physical position of terminals and hardware states, which causes difficulties for terminal extrapolation.
Hence, unlike the antenna and frequency extrapolations, the terminal extrapolation occurs among different parameter subspaces with large differences and it is difficult to realize satisfactory extrapolation performance only through NNs.
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